The present invention relates to borehole resistivity measurement apparatus for measurement of resistivity of sub-surface geological rock formations and more particularly to such measurement using a relatively small diameter well logging tool.
As a background to the present invention it is known that in the process of drilling a well, the fluids used by the drilling process invade the sub surface geological rock formation for a radial distance around the well, displacing the fluids already there. The electrical resistivity of the fluids in the rock pores is the controlling influence on the measurement of the bulk formation resistivity. The resistivity of the drilling fluids is usually different to that of the original formation fluids giving rise to a xe2x80x9cresistivity invasion profilexe2x80x9d radiating outwards away from the well. It is necessary, when evaluating a formation for the presence of hydrocarbon, to know the undisturbed formation resistivity. Any measurement made by a logging tool in the borehole will be perturbed by the invasion and by the borehole itself. It is therefore usually necessary to make two or more measurements which have a different sensitivity to the xe2x80x9cinvaded zonexe2x80x9d in order to be able to calculate the resistivity of the undisturbed formation. It is known to use a laterolog logging tool for this purpose.
The laterolog logging tool measures a series of electrical currents and potentials from which a resistance can be calculated. An array of electrodes confines measured currents into geometrically defined patterns. A knowledge of these patterns, and of the calculated resistances, enables resistivities to be determined. The electrode array is varied depending on the depth of the measurement required. It is also necessary to make the measurements with alternating current, or switched polarity direct current, to avoid polarisation effects arising from making measurements in an ionic fluid.
All the above is well known and described in U.S. Pat. No. 3,772,589.
The known apparatus and method is suitable for boreholes having a standard diameter and for logging tools having a relatively large diameter, eg 4 inches (10.0 cm). With such logging tools correction factors are employed using established correction charts which compensate for fluid present in the borehole between the logging tool and the edge of the borehole.
Such correction charts are, however, only applicable if the distance between the logging tool and the wall of the borehole is relatively short. As the distance grows then the compensation factor increases steeply and compensation becomes impracticable, thereby rendering any measurement unusable.
The present invention relates to relatively narrow diameter logging tools and to improvements in such tools to enable resistivity measurements to be obtained in standard diameter boreholes. Such measurements would not be possible using narrow diameter tools with standard resistivity measurement apparatus, as known from U.S. Pat. No. 3,772 589, because the compensation would be in the upper range of the steep correction curve where the measurement accuracy would not be reliable.
The present invention therefore has as its principal object to provide apparatus for measurement of the resistivity of a geological rock formation using a relatively narrow diameter logging tool.
The new tool is a small diameter, preferably 2xc2xc inches and has a novel array which is designed to deliver a similar performance to that provided by a large diameter tool. In order to achieve this, three fundamental criteria are required to be met.
Firstly, the level of correction required to correct for the presence of the borehole should be little or no greater than that of the larger conventional tools.
Secondly, the array has been developed such that the borehole correction for both measurements (the xe2x80x9cDeepxe2x80x9d and xe2x80x9cShallowxe2x80x9d) is similar, so that the two measurements xe2x80x9ctrackxe2x80x9d as the borehole conditions vary.
Thirdly, the array has also been designed to ensure that the xe2x80x9cvertical resolutionxe2x80x9d i.e. the vertical distance over which the measurement is made, is the same for the two measurements.
The present invention therefore provides logging tool for logging the resistivity of a geological rock information, the logging tool comprising a plurality of electrodes forming part of a switchable circuit that generates currents that are measurable in a said formation for the purpose of determining the resistivity thereof, the plurality of electrodes including a pair of auxiliary electrodes, each said auxiliary electrode being separated into a first electrode portion and a second electrode portion and including switch means that are operable for repeatedly sequentially combining each said first and second electrode portions into a single energizable emitter electrode when said logging tool is used in a deep mode and separating said first and second electrode portions to enable only one of said first or second electrode portions to be energized as an emitter when said logging tool is used in a shallow mode, the operation of the switch means occurring sufficiently rapidly as to provide substantially simultaneous deep and shallow mode measurements.
The present invention also provides a method for operating a logging tool, including a plurality of electrodes forming part of a switchable circuit that generates currents that are measurable in a said formation for the purpose of determining the resistivity thereof, to log the resistivity of a geological rock formation, the method comprising the steps of surveying the formation in a first deep mode and then surveying the formation in a shallow mode and comprising the step of changing the length of an auxiliary electrode that is one of the said plurality of electrodes, when said survey in said shallow mode is conducted from the length of the auxiliary electrode when said deep mode survey is conducted, the said changing step occurring sufficiently rapidly as to provide substantially simultaneous deep and shallow mode measurements.
The present invention also provides a method for operating a logging tool to log the resistivity of a geological rock formation, comprising the steps of surveying the formation in a first deep mode and then surveying the formation in a shallow mode and comprising the step of changing the length of auxiliary electrode as defined when said survey in said shallow mode is conducted from the length of the auxiliary electrode when said deep mode survey is conducted.